The present invention relates to a plasma device and plasma generating method for generating a plasma by an electromagnetic field supplied into a processing vessel by using a slot antenna.
In the manufacture of a semiconductor device or flat panel display, plasma devices are used often to perform processes such as formation of an oxide film, crystal growth of a semiconductor layer, etching, and ashing. Among the plasma devices, a high frequency plasma device is available which supplies a high frequency electromagnetic field into a processing vessel by using a slot antenna and generates a high-density plasma with the electromagnetic field. The high frequency plasma device is characterized in that it can stably generate a plasma even if the pressure of the plasma gas is comparatively low.
When various types of processes are to be performed by using the plasma device, the two-dimensional distribution (to be referred to as “surface distribution” hereinafter) of the plasma on a processing surface of a semiconductor substrate or the like must be uniformed.
As a method of generating a plasma having a uniform surface distribution by using a slot antenna, a high frequency electromagnetic field fed from a high frequency power supply may be converted into a rotating electromagnetic field, and the circularly polarized rotating high frequency electromagnetic field may be supplied to the slot antenna.
FIG. 21 is a view showing an arrangement of a conventional high frequency plasma device. FIG. 21 shows the longitudinal sectional structure of the arrangement of part of the conventional high frequency plasma device.
The conventional plasma device has a bottomed cylindrical processing vessel 111 with an upper opening, a dielectric plate 113 for closing the upper opening of the processing vessel 111, and a radial antenna 130 arranged above the dielectric plate 113 to radiate a high frequency electromagnetic field into the processing vessel 111.
A substrate table 122 is fixed to the bottom of the processing vessel 111, and a substrate 121 as a target object is arranged on the mount surface of the substrate table 122. Exhaust ports 116 for vacuum exhaustion are formed in the bottom of the processing vessel 111, and a nozzle 117 for supplying a plasma gas and process gas is formed in the side wall of the processing vessel 111.
The dielectric plate 113 is made of silica glass or the like. A seal member (not shown) such as an O-ring is interposed between the dielectric plate 113 and processing vessel 111, so that the plasma in the processing vessel 111 will not leak to the outside.
The radial antenna 130 is a kind of slot antennas, and is formed of two parallel circular conductor plates 131 and 132 which form a radial waveguide 133, and a conductor ring 134 which connects the edge portions of the conductor plates 131 and 132. An inlet 135 for introducing the high frequency electromagnetic field into the radial antenna 130 is formed at the center of the conductor plate 132 serving as the upper surface of the radial waveguide 133. A plurality of slots 136 for radiating an electromagnetic field F, which propagates in the radial waveguide 133, into the processing vessel 111 through the dielectric plate 113 are formed in the circumferential direction in the conductor plate 131 serving as the lower surface of the radial waveguide 133, thus forming the antenna surface of the radial antenna 130. The outer portions of the radial antenna 130 and dielectric plate 113 are covered by an annular shield material 112. Thus, the electromagnetic field will not leak to the outside.
In the conventional plasma device, the rotating electromagnetic field is supplied to the radial antenna 130 with the above arrangement.
More specifically, in order to supply a rotating electromagnetic field, the conventional plasma device has a high frequency generator 145 for generating a high frequency electromagnetic field, a rectangular waveguide 143 for guiding the high frequency electromagnetic field output from the high frequency generator 145, a rectangular cylindrical converter 147 for connecting the rectangular waveguide and a cylindrical waveguide, and a circular polarization converter 146 for converting a linearly polarized high frequency electromagnetic field into a rotating electromagnetic field.
As the circular polarization converter 146, for example, one having one or the plurality of sets of axial cylindrical stubs 146A made of conductors on the inner wall of a cylindrical waveguide to oppose each other is used, as shown in FIG. 22(c). The cylindrical stubs 146A are arranged in directions to form 45° with the main direction of the electric field of a TE11-mode electromagnetic field input from the rectangular cylindrical converter 147. When the plurality of sets of cylindrical stubs 146A are arranged, they are provided at an interval of λ/4 (λ is the guide wavelength of the propagating electromagnetic wave) in the axial direction, and convert the TE11-mode high frequency electromagnetic field into a rotating electromagnetic field the main direction of the electric field of which rotates about the axis of the cylindrical waveguide as the center.
In the conventional plasma device with the above arrangement, how the rotating electromagnetic field is supplied will be described as follows with reference to FIG. 22. FIG. 22 includes views schematically showing how the electromagnetic field propagates in the rectangular waveguide 143, rectangular cylindrical converter 147, and circular polarization converter 146. FIG. 22(a) shows the state of the electric field taken along A-A′ of the electromagnetic field which propagates in the rectangular waveguide 143 shown in FIG. 21, FIGS. 22(b), 22(e), and 22(f) each show the state of the electric field taken along an outlet B-B′ of the rectangular cylindrical converter 147, and FIGS. 22(c), 22(d), and 22(g) each show the electric field and the rotating direction of the electromagnetic field propagating in the circular polarization converter 146.
The high frequency electromagnetic field (FIG. 22(a)) which has propagated in the rectangular waveguide 143 from the high frequency generator 145 with the TE10 mode is converted by the rectangular cylindrical converter 147 into the TE11 mode (FIG. 22(b)), and is introduced into the cylindrical waveguide of the circular polarization converter 146. The high frequency electromagnetic field is then converted into a rotating electromagnetic field while propagating in the circular polarization converter 146 (FIG. 22(c)), and is supplied into the radial antenna 130 through the inlet 135 formed at the center of the conductor plate 132.
The rotating electromagnetic field supplied to the radial antenna 130 is, however, partly reflected by the conductor ring 134 located at the end of the radial waveguide 133. The reflected rotating electromagnetic field propagates in the circular polarization converter 146 in the opposite direction while rotating in the same direction (FIG. 22(d)). The reflected electromagnetic field is then reflected at the fixed end of the rectangular cylindrical converter 147 (FIGS. 22(e) and 22(f)), forms a rotating electromagnetic field rotating in the opposite direction to propagate in the circular polarization converter 146 (FIG. 22(g)), and is supplied to the radial antenna 130.
As a result, rotating electromagnetic fields having different phases and rotating directions are supplied to the radial antenna 130 in a mixed state. The polarized wave of the high frequency electromagnetic field at this time forms an ellipse as shown in FIG. 23. This decreases the surface distribution uniformity of the plasma generated in the processing vessel, and nonuniformity occurs in the plasma process particularly at the peripheral portion.
In this manner, if the rotating electromagnetic field converted by the circular polarization converter 146 is merely supplied to the radial antenna 130, it is difficult to obtain the surface uniformity of the plasma distribution due to the influence of the reflected electromagnetic field from the radial antenna 130.